The ever-increasing demand for renewable fuels and chemicals has forced refineries to look to alternative hydrocarbon sources and ways to upgrade and convert these sources or feedstocks into viable products. In particular, refineries need processes to upgrade renewable and non-renewable feedstocks, such as plant oils, algal and microbial oils, waste vegetable oils, yellow and brown grease, tallow, soap stock, pyrolysis oils from plastics or cellulose, and petroleum fractions into high-value, light- and middle-distillate hydrocarbon products. Renewable, waste, and low-cost feedstocks often contain contaminants that must be removed collectively prior to upgrading by conventional refinery processes into clean hydrocarbon fuels or chemicals.
Renewable plant oils typically contain phospholipid compounds or complexes, referred to herein as phospholipids. The phosphorous in phospholipids creates two primary problems for conventional refinery unit operations. First, phosphorus is a nucleating site and catalyst for coke formation. Renewable and petroleum feedstocks that are high in phosphorus cause coking in fired-furnaces and heat exchangers, which greatly increases downtime for decoking and other maintenance operations. Second, phosphorus irreversibly poisons and deactivates catalysts used in hydrotreating, hydrocracking, and hydroisomerization, leading to more frequent and costly catalyst replacement. Catalysts may be protected using guard beds containing alumina or similar high-surface area materials that can sorb low concentrations of metal and phosphorus compounds, but this approach is cost prohibitive for renewable oils containing high levels of phospholipids.
Renewable oils containing phospholipids may be chemically degummed to eliminate phosphorus. Phospholipids contain a diglyceride (two fatty acid chains covalently bonded to a glycerin molecule through ester linkages), a phosphate group (PO43−), and are typically complexed with various organic molecules, such as choline (C5H14NO), ethanolamine, serine, inositol, and the like. Conventional chemical degumming uses phosphoric acid or citric acid to remove phosphorus as phosphatidic acid. Phosphatidic acid includes the two fatty acids and the glycerin backbone from the original, phospholipid. Thus, conventional chemical degumming of a plant or algal oil that is high in phospholipid content results in significant yield loss because the entire phospholipid diglyceride is removed from the treated oil.
Processes for converting renewable oils into renewable, hydrocarbon fuels (as opposed to fatty acid methyl esters or FAME biodiesel) typically hydrotreat the triglyceride feedstock resulting in hydrogenolysis of the glycerin backbone. Partially due to the hydrogen that is required to hydrotreat the glycerin backbone, which produces propane (a low-value byproduct), this process requires up to 100% more hydrogen than is required for fatty acid deoxygenation alone. Hydrolysis or “fat-splitting” processes may be used to produce glycerin and free fatty acids that are used for renewable fuel or chemical production. A widely employed hydrolysis process is the Colgate-Emery process.
The Colgate-Emery process is a continuous-flow, counter-current process that typically operates at 250-260° C. and 725 psig. Oil is fed into the bottom of a splitting tower and demineralized water is fed into the top of the tower. Fatty acids are discharged from the top of the tower and a water-glycerin solution (sweet water) is removed from the bottom of the tower. Processing time is 2 to 3 hours, which requires very large heated pressure vessels for large commercial applications. Several factors limit the performance of a Colgate-Emery process: 1) the need to operate below the glycerin decomposition temperature, which is approximately 290° C.; 2) the need to provide long residence time for hydrolysis and to permit gravity separation of free fatty acid and glycerin-water phases; and 3) the economical tradeoff between operating temperature, pressure, and residence time. Operation of the Colgate-Emery process at higher temperature requires higher pressure and risks decomposition of glycerin due to the long residence time at temperatures near 290° C. The large equipment required makes this process cost prohibitive for alternative fuel production due to the large volumes of oil that must be processed in order to achieve economic viability. Sweet water (a diluted solution of glycerin) may form an emulsion due to the presence of residual free fatty acids and partially hydrolyzed triglycerides. To recover the dilute glycerin product, sweet water typically must settle for up to 24 hours at 80-90° C. with demulsifying agents. Vacuum distillation may also be used to further separate long- and short-chain fatty acids.
Solvent deasphalting is a refinery process for extracting asphaltenes and resins from atmospheric tower bottoms (ATB) or other heavy petroleum fraction to produce deasphalted oil (DAO) that can be used as feed to fluid catalytic cracking or hydrocracking systems. The process consists of contacting the feedstock with a solvent in a counter-current extractor at temperatures and pressures to precipitate the asphaltene and resin fractions that are not soluble in the solvent. Solvents may be low molecular weight paraffins such as propane, butane, pentane, or hexane. The solvent deasphalting process requires a considerable amount of expensive solvent. Solvent recovery is an energy-intensive process. DAO yields are typically only 40-60% and higher yields can only be obtained by sacrificing DAO quality.
Salt compounds also must be limited in crude feedstocks due to corrosion, coking, and catalyst fouling issues that arise when salt compounds are present in the feedstock during feedstock conversion. Conventional desalting processes mix petroleum crude oil and water at elevated temperatures through a mixing valve to form an intimately mixed stream that is then fed to a large oil-water separator. Separation is facilitated by passing high frequency alternating current through the organic phase to cause small water droplets to coalesce. Demulsifying agents are also used to facilitate removal of water. Often, a two-stage desalting system is required. Renewable oils, such as waste vegetable oil, yellow and brown grease, and tallow, are difficult to desalt using conventional petroleum desalters, in part, due to the conductivity of these oils and their potential to form soaps and emulsions.